DESCRIPTION

MasonX::Interp::WithCallbacks subclasses HTML::Mason::Interp in order to provide a Mason callback system built on Params::CallbackRequest. Callbacks may be either code references provided to the new() constructor, or methods defined in subclasses of Params::Callback. Callbacks are triggered either for every request or by specially named keys in the Mason request arguments, and all callbacks are executed at the beginning of a request, just before Mason creates and executes the request component stack.

This module brings support for a sort of plugin architecture based on Params::CallbackRequest to Mason. Mason then executes code before executing any components. This approach allows you to carry out logical processing of data submitted from a form, to affect the contents of the Mason request arguments (and thus the %ARGS hash in components), and even to redirect or abort the request before Mason handles it.

Much of the documentation here is based on that in Params::CallbackRequest, although it prefers using HTML form fields for its examples rather than Perl hashes. But see the Params::CallbackRequest documentation for the latest on its interface.

JUSTIFICATION

Why would you want to do this? Well, there are a number of reasons. Some I can think of offhand include:

Stricter separation of logic from presentation

Most application logic handled in Mason components takes place in <%init> blocks, often in the same component as presentation logic. By moving the application logic into Perl modules and then directing Mason to execute that code as callbacks, you obviously benefit from a cleaner separation of application logic and presentation.

Widgitization

Thanks to their ability to preprocess arguments, callbacks enable developers to develop easier-to-use, more dynamic widgets that can then be used in any and all Mason component, or even with other templating systems. For example, a widget that puts many related fields into a form (such as a date selection widget) can have its fields preprocessed by a callback (for example, to properly combine the fields into a unified date field) before the Mason component that responds to the form submission gets the data. See Params::Callback for an example solution for this very problem.

Shared Memory

Callbacks are just Perl subroutines in modules, and are therefore loaded at server startup time in a mod_perl environment. Thus the memory they consume is all in the Apache parent process, and shared by the child processes. For code that executes frequently, this can be much less resource-intensive than code in Mason components, since components are loaded separately in each Apache child process (unless they're preloaded via the preloads parameter to the HTML::Mason::Interp constructor).

Performance

Since they're executed before Mason creates a component stack and executes the components, callbacks have the opportunity to short-circuit the Mason processing by doing something else. A good example is redirection. Often the application logic in callbacks does its thing and then redirects the user to a different page. Executing the redirection in a callback eliminates a lot of extraneous processing that would otherwise be executed before the redirection, creating a snappier response for the user.

Testing

Mason components are not easy to test via a testing framework such as Test::Harness. Subroutines in modules, on the other hand, are fully testable. This means that you can write tests in your application test suite to test your callback subroutines.

And if those aren't enough reasons, then just consider this: Callbacks are just way cool.

USAGE

MasonX::Interp::WithCallbacks uses Params::CallbackRequest for its callback architecture, and therefore supports its two different types of callbacks: those triggered by a specially named key in the Mason request arguments hash, and those executed for every request.

Argument-Triggered Callbacks

Argument-triggered callbacks are triggered by specially named request argument keys. These keys are constructed as follows: The package name followed by a pipe character ("|"), the callback key with the string "_cb" appended to it, and finally an optional priority number at the end. For example, if you specified a callback with the callback key "save" and the package key "world", a callback field might be added to an HTML form like this:

<input type="button" value="Save World" name="world|save_cb" />

This field, when submitted to the Mason server, would trigger the callback associated with the "save" callback key in the "world" package. If such a callback hasn't been configured, then Params::CallbackRequest will throw a Params::CallbackReuest::Exception::InvalidKey exception. Here's how to configure a functional callback when constructing your MasonX::Interp::WithCallbacks object so that that doesn't happen:

With this configuration, the request argument created by the above HTML form field will trigger the execution of the &My::World::save subroutine.

Functional Callback Subroutines

Functional callbacks use a code reference for argument-triggered callbacks, and Params::CallbackRequest executes them with a single argument, a Params::Callback object. Thus, a callback subroutine will generally look something like this:

sub foo {
my $cb = shift;
# Do stuff.
}

The Params::Callback object provides accessors to data relevant to the callback, including the callback key, the package key, and the request arguments (or parameters). It also includes redirect() and abort() methods. See the Params::Callback documentation for all the goodies.

Note that all callbacks are executed in a eval {} block, so if any of your callback subroutines die, Params::CallbackRequest will throw an Params::CallbackRequest::Exception::Execution exception If you don't like this, use the cb_exception_handler parameter to new() to install your own exception handler.

Object-Oriented Callback Methods

Object-oriented callback methods are defined in subclasses of Params::Callback. Unlike functional callbacks, they are not called with a Params::Callback object, but with an instance of the callback subclass. These classes inherit all the goodies provided by Params::Callback, so you can essentially use their instances exactly as you would use the Params::Callback object in functional callback subroutines. But because they're subclasses, you can add your own methods and attributes. See Params::Callback for all the gory details on subclassing, along with a few examples. Generally, callback methods will look like this:

sub foo : Callback {
my $self = shift;
# Do stuff.
}

As with functional callback subroutines, method callbacks are executed in a eval {} block. Again, see the cb_exception_handler parameter to install your own exception handler.

Note: It's important that you use any and all MasonX::Callback subclasses before you use MasonX::Interp::WithCallbacks or use Params::CallbackRequest. This is to get around an issue with identifying the names of the callback methods in mod_perl. Read the comments in the MasonX::Callback source code if you're interested in learning more.

The Package Key

The use of the package key is a convenience so that a system with many functional callbacks can use callbacks with the same keys but in different packages. The idea is that the package key will uniquely identify the module in which each callback subroutine is found, but it doesn't necessarily have to be so. Use the package key any way you wish, or not at all:

But note that if you don't use the package key at all, you'll still need to provide one in the parameters to be submitted to exec() By default, that key is "DEFAULT". Such a callback field in an HTML form would then look like this:

<input type="button" value="Save World" name="DEFAULT|save_cb" />

If you don't like the "DEFAULT" package name, you can set an alternative default using the default_pkg_name parameter to new():

Then, of course, any callbacks without a specified package key of their own will then use the custom default:

<input type="button" value="Save World" name="MyPkg|save_cb" />

The Class Key

The class key is essentially a synonym for the package key, but applies more directly to object-oriented callbacks. The difference is mainly that it corresponds to an actual class, and that all Params::Callback subclasses are required to have a class key; it's not optional as it is with functional callbacks. The class key may be declared in your Params::Callback subclass like so:

If no class key is explicitly defined, Params::Callback will use the subclass name, instead. In any event, the register_callback() method must be called to register the subclass with Params::Callback. See the Params::Callback documentation for complete details.

Priority

Sometimes one callback is more important than another. For example, you might rely on the execution of one callback to set up variables needed by another. Since you can't rely on the order in which callbacks are executed (the Mason request arguments are stored in a hash, and the processing of a hash is, of course, unordered), you need a method of ensuring that the setup callback executes first.

In such a case, you can set a higher priority level for the setup callback than for callbacks that depend on it. For functional callbacks, you can do it like this:

In these examples, the "setup" callback has been configured with a priority level of "3". This ensures that it will always execute before the "save" callback, which has the default priority of "5". This is true regardless of the order of the fields in the corresponding HTML::Form:

Despite the fact that the "setup" callback field appears after the "save" field (and will generally be submitted by the browser in that order), the "setup" callback will always execute first because of its higher priority.

Although the "save" callback got the default priority of "5", this too can be customized to a different priority level via the default_priority parameter to new() for functional callbacks and the default_priority to the class declaration for object-oriented callbacks For example, this functional callback configuration:

Causes the "save" callback to execute before the "setup" callback by overriding the "save" callback's priority to level "2". Of course, any other form field that triggers the "save" callback without a priority override will still execute "save" at its configured level.

Request Callbacks

Request callbacks come in two separate flavors: those that execute before the argument-triggered callbacks, and those that execute after the argument-triggered callbacks. All of them execute before the Mason component stack executes. Functional request callbacks may be specified via the pre_callbacks and post_callbacks parameters to new(), respectively:

In these examples, the translate() and foobarate() subroutines or methods will execute (in that order) before any argument-triggered callbacks are executed (none will be in these examples, since none are specified).

Conversely, the escape() and negate() subroutines or methods will be executed (in that order) after all argument-triggered callbacks have been executed. And regardless of what argument-triggered callbacks may be triggered, the request callbacks will always be executed for every request.

Although they may be used for different purposes, the pre_callbacks and post_callbacks functional callback code references expect the same argument as argument-triggered functional callbacks: a Params::Callback object:

sub foo {
my $cb = shift;
# Do your business here.
}

Similarly, object-oriented request callback methods will be passed an object of the class defined in the class key portion of the callback trigger -- either an object of the class in which the callback is defined, or an object of a subclass:

sub foo : PostCallback {
my $self = shift;
# ...
}

Of course, the attributes of the Params::Callback or subclass object will be different than in argument-triggered callbacks. For example, the priority, pkg_key, and cb_key attributes will naturally be undefined. It will, however, be the same instance of the object passed to all other functional callbacks -- or to all other class callbacks with the same class key -- in a single request.

Like the argument-triggered callbacks, request callbacks are executed in a eval {} block, so if any of them dies, an Params::CallbackRequest::Exception::Execution exception will be thrown. Use the cb_exception_handler parameter to new() if you don't like this.

INTERFACE

Parameters To The new() Constructor

In addition to those offered by the HTML::Mason::Interp base class, this module supports a number of its own parameters to the new() constructor based on those required by Params::CallbackRequest. Each also has a corresponding httpd.conf variable as well, so, if you really want to, you can use MasonX::Interp::WithCallbacks right in your httpd.conf file:

The parameters to new() and their corresponding httpd.conf variables are as follows:

callbacks

Argument-triggered functional callbacks are configured via the callbacks parameter. This parameter is an array reference of hash references, and each hash reference specifies a single callback. The supported keys in the callback specification hashes are:

cb_key

Required. A string that, when found in a properly-formatted Mason request argument key, will trigger the execution of the callback.

cb

Required. A reference to the Perl subroutine that will be executed when the cb_key has been found in a Mason request argument key. Each code reference should expect a single argument: a Params::Callback object. The same instance of a Params::Callback object will be used for all functional callbacks in a single request.

pkg_key

Optional. A key to uniquely identify the package in which the callback subroutine is found. This parameter is useful in systems with many callbacks, where developers may wish to use the same cb_key for different subroutines in different packages. The default package key may be set via the default_pkg_key parameter.

priority

Optional. Indicates the level of priority of a callback. Some callbacks are more important than others, and should be executed before the others. Params::CallbackRequest supports priority levels ranging from "0" (highest priority) to "9" (lowest priority). The default priority for functional callbacks may be set via the default_priority parameter.

The <callbacks> parameter can also be specified via the httpd.conf configuration variable MasonCallbacks. Use PerlSetVar to specify several callbacks; each one should be an evalable string that converts into a hash reference as specified here. For example, to specify two callbacks, use this syntax:

Note that the evalable string must be entirely on its own line in the httpd.conf file.

pre_callbacks

This parameter accepts an array reference of code references that should be executed for every request before any other callbacks. They will be executed in the order in which they're listed in the array reference. Each code reference should expect a single Params::Callback argument. The same instance of a Params::Callback object will be used for all functional callbacks in a single request. Use pre-argument-triggered request callbacks when you want to do something with the arguments submitted for every request, such as convert character sets.

The <pre_callbacks> parameter can also be specified via the httpd.conf configuration variable MasonPreCallbacks. Use multiple PerlAddVar to add multiple pre-request callbacks; each one should be an evalable string that converts into a code reference:

This parameter accepts an array reference of code references that should be executed for every request after all other callbacks have been called. They will be executed in the order in which they're listed in the array reference. Each code reference should expect a single Params::Callback argument. The same instance of a Params::Callback object will be used for all functional callbacks in a single request. Use post-argument-triggered request callbacks when you want to do something with the arguments submitted for every request, such as HTML-escape their values.

The <post_callbacks> parameter can also be specified via the httpd.conf configuration variable MasonPostCallbacks. Use multiple PerlAddVar to add multiple post-request callbacks; each one should be an evalable string that converts into a code reference:

An array reference listing the class keys of all of the Params::Callback subclasses containing callback methods that you want included in your MasonX::Interp::WithCallbacks object. Alternatively, the cb_classes parameter may simply be the word "ALL", in which case all Params::Callback subclasses will have their callback methods registered with your MasonX::Interp::WithCallbacks object. See the Params::Callback documentation for details on creating callback classes and methods.

Note: Be sure to use MasonX::Interp::WithCallbacks or use Params::CallbackRequestonly after you've used all of the Params::Callback subclasses you need or else you won't be able to use their callback methods.

The <cb_classes> parameter can also be specified via the httpd.conf configuration variable MasonCbClasses. Use multiple PerlAddVar to add multiple callback class keys. But, again, be sure to load MasonX::Interp::WithCallbacks or Params::CallbackRequest only after you've loaded all of your MasonX::Callback handler subclasses:

The priority level at which functional callbacks will be executed. Does not apply to object-oriented callbacks. This value will be used in each hash reference passed via the callbacks parameter to new() that lacks a priority key. You may specify a default priority level within the range of "0" (highest priority) to "9" (lowest priority). If not specified, it defaults to "5".

Use the MasonDefaultPriority variable to set the the default_priority parameter in your httpd.conf file:

PerlSetVar MasonDefaultPriority 3

default_pkg_key

The default package key for functional callbacks. Does not apply to object-oriented callbacks. This value that will be used in each hash reference passed via the callbacks parameter to new() that lacks a pkg_key key. It can be any string that evaluates to a true value, and defaults to "DEFAULT" if not specified.

Use the MasonDefaultPkgKey variable to set the the default_pkg_key parameter in your httpd.conf file:

PerlSetVar MasonDefaultPkgKey CBFoo

ignore_nulls

By default, Params::CallbackRequest will execute all request callbacks. However, in many situations it may be desirable to skip any callbacks that have no value for the callback field. One can do this by simply checking $cb->value in the callback, but if you need to disable the execution of all callbacks, pass the ignore_nulls parameter with a true value. It is set to a false value by default.

Use the MasonIgnoreNulls variable to set the the ignore_nulls parameter in your httpd.conf file:

PerlSetVar MasonIgnoreNulls 1

cb_exception_handler

When Params::CallbackRequest encounters an exception during the execution of callbacks, it normally calls Params::CallbackRequest::Exceptions::rethrow_exception to handle the exception. But if you throw your own exceptions in your callbacks, and want to handle them differently (say, to handle them and then let the request continue), pass the cb_exception_handler parameter a code reference to do what you need.

Use the MasonCbExceptionHandler variable to set the cb_exception_handler parameter in your httpd.conf file:

MasonCbExceptionHandler "sub {...}"

Note: This is the only parameter that differs in name from the same parameter to Params::CallbackRequest->new. This is so that it can be easily distinguished from the possible addition of a exception_handler parameter to a future version of Mason.

Accessor Methods

All of the above parameters to new() are passed to the Params::CallbackRequest constructor and deleted from the MasonX::Interp::WithCallbacks object. MasonX::Interp::WithCallbacks then contains a Params::CallbackRequest object that it uses to handle the execution of all callbacks for each request.

Notes

The Params::CallbackRequest notes interface remains available via the notes() method of both Params::CallbackRequest and Params::Callback. Notes stored via this interface will be copied to the HTML::Mason::Request notes() interface before the execution of the request, and continue to be available for the lifetime of the Mason request via $interp->cb_request->notes. Notes will be cleared out at the end of the request, just as with $r->pnotes.

SUPPORT

This module is stored in an open GitHub repository. Feel free to fork and contribute!

SEE ALSO

Params::CallbackRequest handles the processing of the Mason request arguments and the execution of callbacks. See its documentation for the most up-to-date documentation of the underlying callback architecture.

Params::Callback objects get passed as the sole argument to all functional callbacks, and offer access to data relevant to the callback. Params::Callback also defines the object-oriented callback interface, making its documentation a must-read for anyone who wishes to create callback classes and methods.